1. Field of the Invention
The present invention relates to an image sensor IC (integrated circuit) used in a device for capturing and transmitting image information, such as a facsimile, an image scanner, and an electronic camera.
2. Description of the Related Art
FIG. 3 is an exemplary circuit diagram for showing an image sensor of a related art. In a sensor circuit 10 of a MOS image sensor, a reset transistor 11 serving as a switching element for resetting a photodiode 12 to an appropriate voltage, and an amplifier circuit 13 for amplifying light induced charges accumulated in the photodiode 12 are connected to the photodiode 12 including a PN junction.
Optical information can be obtained continuously through following three operations: a reset operation in which the reset transistor 11 is turned on so as to reset the photodiode 12 to a reset voltage to a satisfactory extent; an accumulation operation in which the reset transistor 11 is turned off so as to accumulate light induced charges in the photodiode 12 for a predetermined period of time; and a read operation in which the amplifier circuit 13 is turned on so as to amplify the light induced charges accumulated in the photodiode 12 to read.
Temporal storage of amplified signal may also be performed in the read operation by using a holding circuit 20 including a capacitive element 21 and two switching transistors (22A and 22B). The switching transistor 22A is turned on during the read operation, and the signal is stored in the storage capacitor 21 as an electrical charge by the amplifier circuit 13. The switching transistor 22A is then turned off and the switching transistor 22B is later turned on after an arbitrary holding time, thereby permitting the signal read from the storage capacitor 21.
Separate reading of the signal from the holding circuit in an arbitrary order is also possible after a series of operations, that is, the reset operation, the accumulation operation, and the read operation are collectively performed with respect to a plurality of photodiodes.
In these procedures, photoelectric conversion, whose characteristic is one of most the important characteristics in the photodiode, is performed according to an intensity of incident light to the photodiode 12.
In order to improve the photoelectric conversion characteristic, a photoelectric conversion element is disclosed which is capable of suppressing generation of a defect in a semiconductor region in which a depletion layer is formed in the photodiode 12 (for example, see JP 2004-312039 A (FIG. 24).
In the image sensor IC having a plurality of pixels arranged in one IC chip, however, a problem arises in that it leads to a variation of the photoelectric conversion characteristic due to change in intensity of incident light caused by a variation in thickness of a protective film formed on top of each of the plurality of photodiodes 12 which forms the pixels.                Though a countermeasure for the problem has been proposed in which planarization is performed after formation of the protective film, and in which a second protective film is formed to obtain further uniformity in the film thickness, problems still remain in that, for example, the number of process steps increases, which results in an increase in manufacturing costs, and sufficient uniformity cannot still be obtained.        